This invention relates to methods and apparatus for washing raw coal and more particularly to methods and apparatus which has the ability for recovering coal fines down to 200 mesh without the addition of costly chemicals and the requirement of costly equipment.
In this period of critical energy shortages, the nations attention is being directed toward the development of alternate or plentiful sources of energy. One particular source of energy although not new, is becoming of great significance. This energy is, of course, coal. However, before coal can be used as a primary fuel in the generation of electrical energy and other industrial processes it requires treatment. For example, raw coal straight out of a mine often contains significant amounts of contaminant material such as slate, rocks, and other debris. Carefully mining only the center of a coal seam to eliminate or reduce most of the debris or contaminant materials is one approach, but this is not really satisfactory, as it leaves great quantities of coal at the edges of the seam. Therefore, to obtain a clean product, coal washing plants have been developed which remove the contaminant materials and debris from the coal. Unfortunately, the techniques of washing coal in the past resulted in significant loss of coal "fines". That is, coal material below 28 mesh or about 595 microns. Unfortunately, the objections to earlier available coal washing techniques is not only with respect to the loss of coal, but washing of such coal with water typically requires the use of a slurry or settling pond which in itself presents significant environmental dangers. Furthermore, it will be appreciated that although settling ponds may offer a solution (although unattractive) such ponds may typically require several acres for a large coal washing operation. Such a large amount of acreage of land may be available adjacent coal mines in parts of the western states, but it will be appreciated that most coal mining in the eastern states takes place in the mountains. Most eastern coal mines simply are not surrounded by several acres of suitable flat land for a settling pond. Thus, the settling ponds are not satisfactory in many situations because of large land requirements and the potential environmental dangers. Because of this problem, there have been in the past developed very large and costly coal washing plants. These plants, however, may typically run between 10 and 30 million dollars per installation. In addition, these plants are complex and require continuous and significant maintenance as well as continuous addition of costly chemicals such as magnatite to clean and clarify the water and remove the coal fines.
Although there are a great number of patents related to different components of a coal washing plant and for different types of coal washing plants, a typical example is U.S. Pat. No. 1,656,271 issued to C. R. Downs et al on Jan. 17, 1928. The apparatus of this patent illustrates various of the screens and settling tanks which are typically used in coal washing plants. This particular patent uses a process which requires a frothing agent for purposes of recovering the smaller coal. Unfortunately, the patent discloses the use of oil as the frothing agent, and with todays cost of oil this is completely unacceptable. According to the Downs technique, light weight coal fines are suspended and float with the oil to the top while the heavy weight debris, slate, and other materials settle to the bottom providing separation.
U.S. Pat. No. 4,040,961 issued to R. F. Davis, Jr. et al on Aug. 9, 1977, illustrates a more modern plant for the separating of solids from a liquid, and is particularly useful in separating coal and coal fines from a washing liquid. However, this patent also discloses the use of froth flotation which requires the use of significant amounts of chemicals for floating the light weight coal or coal particles away from the heavy weight solid debris and contaminant materials. The Davis patent is in no way capable of carrying out a complete washing cycle of the coal such that it also recovers coal fines down to 200 mesh, clarifies the water and eliminates need for a slurry pit.
Still another U.S. Pat. No. 4,104,128 issued to Paul B. Faber on Aug. 1, 1978, illustrates a complex apparatus and method for recovering coal fines. Although the Faber patent has the ability to recover exceedingly small coal fines, the process is unusually complex and expensive, and is incidental to a self contained system for charging preheated coal into coke ovens.
Various components are typically necessary in washing coal plants which are suitable for recovering ultra coal fines down to 100 mesh. However, none of these components themselves have the capacity for both recovering coal fines and clarifying water. As an example, U.S. Pat. No. 4,128,474 issued to Robert E. Ennis on Dec. 5, 1978 and assigned to Linatex Corporation of America, discloses a wet mechanical process for cleaning, upgrading, and dewatering fine coal. The process provides for forming an aqueous feed slurry of fine coal and its associated contaminant particles wherein all particles have a particle portion size of less than about 6 milimeters. The feed slurry is separated into coal slurry and refuse slurry portions in a spiral gravity concentrator by removing contaminates having a particle size greater than about 0.15 milimeters or 100 mesh. The concentrated coal slurry is then fed to a hydrocyclone separator where all of the ultrafine silt material having a particle size of less than 100 mesh is removed and the coal particles of 6 milimeters to 100 mesh is accumulated and dewatered. Although the apparatus described in this patent is certainly useful in the recovery of coal, it is still not a complete system which removes coal fines down to 200 mesh, reduces the amount of liquid necessary, eliminates the need for a slurry pit, and requires a building no greater than 40 feet by 60 feet.
Other particle separators such as that described in U.S. Pat. No. 3,608,717 and issued to David G. Strubel disclose various other techniques for separating particles of different specific gravities and absorption characteristics in a heavy liquid medium. The Strubel patent is particularly suitable for separating and recovering tobacco fines which would otherwise be lost.
Thus, it can be seen that although varied techniques have been attempted to wash coal and recover coal fines, they all have their problems such as cost or the inability to recover the ultrafine coal. Others require the huge amounts of additional costly chemicals, while still others require that great areas of land be available for slurry or settlement pits. Unlike, the present invention, none of the prior art techniques provide both method and apparatus which are both inexpensive and capable of recovering coal fines down to 200 mesh while at the same time eliminating slurry pits, and the need for huge amounts of water and space.
Therefore, it is an object of this invention to provide simple and inexpensive methods and apparatus for removing contaminant materials from raw coal.
It is still another object of this invention to provide method and apparatus for recovering ultrafine particles of coal which would otherwise be lost.
It is yet another object of this invention to provide method and apparatus for washing and recovering coal fines which is both inexpensive and which eliminates the need for slurry or settling ponds.
Another object of this invention is to clean slurry obtained from washing coal such that water can be reused.
It is still another object of this invention to provide methods and apparatus for washing and recovering coal fines which require minimum space and minimum maintenance.
To accomplish the above mentioned objects as well as other objects which will become evident from the following drawings and detailed description, the present invention provides a method and apparatus which processes the slurry, effluent, and the wet coal obtained when raw coal has been processed through a first and simple washing cycle. Typically, such a washing cycle may be of any suitable method known to those skilled in the art such as a barrel washer capable of recovering coal down to approximately 28 mesh and which has a resultant slurry containing a first group of coal and contaminant particles smaller than 28 mesh. In addition to the slurry, there is also typically an effluent which is comprised of larger groups of contaminate particles down to 28 mesh. The separation and recovery of smaller coal fines according to this invention is preferably accomplished by a cyclone or contrifugal separator which produces water and coal from said first group of particles and a second effluent comprised of water and contrifugal materials from said first group of particle. The resulting water and coal is then processed to obtain wet second coal having a selected size of between about 28 and 80 mesh, and a resulting second slurry which has a group of particles smaller than the particles of the first slurry. This processing step typically will include screening the water and the coal such that the larger particles of coal are recovered and the smaller particles of coal passes through with the water to form the slurry. The second or resulting slurry is then itself separated by means of a hydrocyclone separator or centrifugal separator to recover third coal fines between about 80 and 200 mesh. The cyclone separator also produces an effluent which is comprised of water, coal smaller than the 200 mesh, and contaminant materials. Thus, there has been described a process by which a slurry having coal fines of 28 mesh and smaller is processed to recover coal down to 200 mesh with a resulting effluent of contaminant particles and coal smaller than 200 mesh. The various effluents resulting from the original washing process and the processing to recover the coal between 28 by 200 mesh can itself be processed by separating the effluent into its component parts of water and contaminant materials. Screening of the effluent material from the first washing process results in large contaminant particles and a effluent of smaller size. An effluent having smaller contaminant particles may then be centrifuged to obtain solids and effluents containing still smaller solids and water. The smallest size of contaminant materials contained in the water are then recovered by adding a binding agent such as a polymer which collects and binds the ultrafine particles together so that they may be separated from the liquid thereby resulting in clarified water and contaminant solids which may be readily disposed of in any known manner.
Accordingly, the above mentioned objects and subsequent description will be more readily understood by reference to the following drawings.